A sensor is an object whose purpose is to detect events or changes in its environment, and then provide a corresponding output. A sensor can be a type of a transducer and may provide various types of output, but typically uses electrical or optical signals. Sensors are used in everyday objects such as touch-sensitive elevator buttons (tactile sensor) and lamps which dim or brighten by touching the base, etc. Most presently used sensors need a power source, e.g., a battery or power supply from an electrical outlet, to perform their functions.
A radio-frequency identification (RFID) system can be used to track various types of events using one or more wireless means. RFID is the wireless use of electromagnetic fields to transfer data, for the purposes of automatically identifying and tracking tags attached to objects. The tags contain electronically stored information. Some tags are powered by electromagnetic induction from magnetic fields produced near an RFID reader. Some types of RFID systems have a local power, source such as a battery, and may operate at hundreds of meters from the reader. Other types of tags are passive, e.g., collect energy from the interrogating radio waves and use them to transmit signals. RFID tags are used in many industries. For example, an RFID tag attached to an automobile during production can be used to track its progress through the assembly line; RFID-tagged pharmaceuticals can be tracked through warehouses; implanting RFID microchips in livestock and pets allows positive identification of animals; implanting RFID tags in clothing or other products allow them to be tracked in shopping malls: etc.
Regardless of whether RFID tags are passive or active, the RFID tags described above are typically used for just reading data. They may not be used as sensors to detect events. The RFID tags cannot be turned off or on, e.g., based on the events happening in the environment they are used, to detect the events.